A pulmonary embolism is a blood clot that has lodged in one of the pulmonary arteries between the heart and the lungs. Typically, the clot formed in one of the major veins such as one of the leg veins, broke lose from its point of formation, traveled through the veins to the heart and was pumped by the heart into the pulmonary arteries. The pulmonary arteries are a tree-like structure extending from the heart to the left and right lungs. The pulmonary arteries continually decrease in size from the heart to the lungs and eventually the blood clot lodges at a point where the lumen of the artery is no longer wider than the blood clot. FIG. 1 illustrates an embolism 100 that has lodged at a point in the artery 110 where the lumen is no longer wide enough to let it pass.
In the lungs, carbon dioxide carried by the blood from the pulmonary arteries is exchanged for oxygen. The oxygen-enriched blood is returned to the heart via the pulmonary veins for distribution to the rest of the body. The pulmonary veins are a tree-like structure similar to the pulmonary arteries; and most images of the pulmonary veins as seen in a typical CT image section are indistinguishable from images of the pulmonary arteries.
To locate a pulmonary embolism using CT, a contrast agent is injected into the blood stream and a CT scan is made of the pulmonary region. The CT scan produces a series of images of sections of the pulmonary region, perhaps as many as 200 such images. The embolism will appear in the series of CT images as a point in a vessel structure that has a brighter region on one side of the point than on the other side. Four such CT images are shown in FIGS. 2A-2D. In each case the pulmonary embolism is circled. The change in contrast arises because the embolism prevents the contrast agent from flowing beyond the point in the artery where the embolism has lodged.
It is advantageous to be able to read a series of CT sections using computer-aided detection techniques. In present practice, such a reading provides “a second look” or a check on an independent reading made by a radiologist. Such a reading is performed by the computer by using a region growing technique to proceed along the pulmonary arteries from the heart to the lungs looking for a point where there is an abrupt change in the gray scale of the image. Unfortunately, with presently available algorithms it is sometimes difficult for the computer to distinguish between the pulmonary arteries that must be read and the pulmonary veins that do not have to be read. As a result, pulmonary veins may be incorrectly found to be arteries (false positives) by CAD and are read anyway with considerable increase in the total time required to complete a reading.